LocalSingleBlockElim: Add store-store elimination

Eliminate unused store to variable if followed by store to same
variable in same block.

Most significantly, this cleans up stores made unused by this pass.
These useless stores can inhibit subsequent optimizations, specifically
LocalSingleStoreElim. Eliminating them makes subsequent optimization more
effective.

The main effect of this pass is to simplify the work done by the SSA
rewriter.  It catches many local loads/stores that help speeding up the
work done by the main rewriter.